1. Field of the Invention
The invention relates to ribbon assemblies which provide the functional capability of break-out of the individual coated optical glass fibers. Also provided are radiation-curable ink compositions that are suitable for use in forming the ribbon assemblies.
2. Description of Related Art
Optical glass fibers are generally coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which is in direct contact with the glass is called the inner primary coating and the overlaying coating(s) is called the outer primary coating.
The inner primary coating is usually a relatively soft coating providing environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending. Microbending in the coated fiber can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating(s), which is on the exposed surface of the coated fiber, is typically a relatively harder coating designed to provide a desired resistance to physical handling forces, such as those encountered when the fiber is cabled.
For the purpose of multi-channel transmission, ribbon assemblies containing a plurality of coated optical fibers have been used. A typical ribbon assembly comprises a plurality of coated optical glass fibers which are bonded together in a matrix material. For example, the matrix material can encase the optical glass fibers, or the matrix material can edge-bond the optical glass fibers together. Ribbon assemblies provide a modular design which simplifies the construction, installation and maintenance of optical glass fibers by eliminating the need to handle individual optical glass fibers.
Coated optical glass fibers for use in ribbon assemblies are usually coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to facilitate identification of the individual coated optical glass fibers. Thus, the matrix material which binds the coated optical glass fibers together contacts the outer ink layer if present, or the colored outer primary coating.
Ink coatings usually have a thickness of about 3 to about 10 microns and are formed from a pigment dispersed within a UV curable carrier system. The UV curable carrier system contains a UV curable oligomer or monomer that is liquid before curing to facilitate application of the ink composition to the optical glass fiber, and then a solid after being exposed to UV radiation. In this manner, the UV curable ink composition can be applied to a coated optical glass fiber in the same manner as the inner primary and outer primary coatings are applied.
As the demand for coated optical glass fibers has increased, manufacturers must respond by adding more fiber drawing production lines and by attempting to increase the linear line speeds of the existing fiber drawing production lines. In the latter case, one factor which will determine the upper limit for the line speed will be the curing rate characteristics of the radiation-curable ink composition, for a given radiation source and intensity.
If the line speed is increased to the extent that cure rate time requirements of the radiation-curable ink composition are not provided, the radiation-curable ink composition will not have received a sufficient amount of radiation to cause complete cure, or cross-linking, of the radiation-curable ink composition. The production linear line speed is generally inversely related to the amount of radiation striking the optical glass fiber. That is, as the production line speed is increased the amount of radiation exposure to the radiation-curable ink composition during the production process will necessarily decrease for a given radiation source. Incomplete cure of the radiation-curable ink composition is undesirable and must be avoided because then the desired properties of the incompletely cured ink coating may not be achieved and/or the incompletely cured ink coating may retain tackiness (giving problems in subsequent handling) or a malodorous odor may be present, and there may also be an increase in the extractables (undesirable) in the supposedly-cured ink coating.
In general, radiation-curable ink coating compositions can cure at a significantly slower rate than radiation-curable outer primary coating compositions. It is believed that the pigments present in ink compositions contribute to the slower cure speed of ink coatings. Thus, there is a need for improving the cure speed of the ink coating.
While the ink composition must have a very fast cure speed to ensure complete cure of the ink coating on the high speed drawing tower, the increase in cure speed should not come at the expense of other important properties of the ink coating, such as providing suitable break-out performance. Break-out performance is the ability of the ink coating to separate from the matrix material without separating the ink layer from the outer primary coating to provide mid-span access or end-access to the individual coated optical glass fibers contained within the ribbon-assembly. Mid-span access is access to the individual coated optical glass fibers at a portion of the ribbon assembly between the ends of the ribbon assembly and end-access is access to the individual coated optical glass fibers at a terminus of the ribbon assembly. Therefore, there is a need for a radiation-curable ink composition that exhibits adaptable adhesion properties to provide an adhesion between the outer primary coating and the ink coating that is greater than the adhesion between the ink coating and the matrix material to provide suitable break-out performance.
In addition, ink compositions should not contain ingredients that can migrate to the surface of the optical glass fiber and cause corrosion. The ink composition should also not contain ingredients which can cause instability in the protective coatings or matrix material. Ink coatings for optical glass fibers should be color fast for decades, not cause attenuation of the signal transmission, be impervious to cabling gels and chemicals, and allow sufficient light penetration for fiber core alignment.
From the above, it is clear that optical glass fiber technology places many unique demands on radiation-curable ink compositions which more conventional technologies, such as printing inks, do not.
U.S. Pat. No. 4,900,126 discloses a ribbon assembly in which the bond interface between the matrix material and the colorant material is weaker than the bond interface between the colorant material and the outermost coating on the optical glass fiber. To provide this property, the matrix material cannot be identical to the secondary coating on the optical glass fiber. Furthermore, a release agent must be added to the outer surface of the coated optical glass fibers prior to application of the matrix material. This method of providing break-out performance is undesirable because of the extra step of applying the release agent must conducted and the release agent may undesirably cause delamination of the matrix material from the coated optical glass fibers.
Published Japanese Patent Application No. H1-152405 discloses a radiation-curable ink composition containing an organic polysiloxane compound. The polysiloxane compound provides the ink coating with the ability to separate more easily from the matrix material in a ribbon assembly.
Published Japanese Patent Application No. 64-22976 discloses radiation-curable ink compositions containing specific radiation-curable oligomers. The ink composition provides an ink coating having adhesion to the outer primary coating which is separable from the matrix material in a ribbon assembly.
There is a need for a ribbon assembly which is capable of providing break-out of the individual coated optical glass fibers, without requiring the use of a release agent.
Usually ink compositions must be cured in an inert atmosphere, i.e. under nitrogen or other inert gasses. Providing inert atmospheres on optical glass fiber drawing towers is expensive. Thus, a radiation-curable ink composition which exhibits a high cure speed in the presence of air would provide significant advantages over ink-compositions that must be cured in an inert atmosphere.